The present invention concerns the use of a mineral filled bimodal polyethylene (PE) for making inner liners of foam insulated refrigerators and freezers.
It is known to prepare inner liners of cabinets of household refrigerators and freezers by using polystyrene (PS), blends of PS with other polymers or coated PS-materials. The insulation layer of such devices generally consists of polyurethane (PU) foam which fills the space between their inner liner and their housing. In order to ensure the foam's good adhesion surface modification of the shell material is generally necessary.
DE-A-19712140 discloses a method for producing a refrigerator or freezer cabinet whose inner and outer shells are thermoformed from a single sheet of material whereby a seamless transition between inner and outer shell is provided. The material is a coextruded platine which consists of a polystyrene layer covered with a layer made of a polystyrene-polyethylene blend.
Such polystyrene based materials offer a good thermoformability, a low shrinkage factor and a good scratch resistance as well as good gloss properties, but are prone to environmental stress cracking. Environmental stress cracking (ESC) is the premature cracking of a polymer material due to the combined action of stress and a fluid. The need for regular cleaning of a refrigerator's inner compartment leads to the inner liner's deterioration by ESC as detergents promote the material's crazing and solvent plasticization. Areas around the inner liners' holes, which are necessary for the foaming process in order to allow the escape of propellants, are most susceptible to ESC as they offer fluids and detergents the opportunity to enter the insulation layer and to attack both surfaces of the liners' material.
In the course of thermoforming of inner liners large parts must be heated rather evenly, a procedure which requires a broad processing window of the material in order to minimize waste production. So far, PE is not suited for thermoforming inner liners, as the material has a narrow processing window which is technically difficult to provide for parts of the needed dimensions.